1. Field of the Invention
The present invention relates to a wireless device and system. The invention is relevant to multi-hop wireless networks such as, for example, sensor networks, and to their component wireless devices.
2. Description of the Related Art
There are various known methods of implementing a network in which wireless devices that cannot communicate directly can nevertheless communicate in multi-hop fashion by relaying packets through a series of intermediate wireless devices. In the most general method, every wireless device is assumed to be ready at all times to communicate with any other wireless device within communication range, so that any wireless device can act as an intermediate relay node between two neighboring wireless devices. New transmission paths can therefore be set up just by routing packets through existing wireless devices, without the cost of additional wiring that would be incurred in a wired network.
A problem with this general method is that since each wireless device must always be ready to receive signals from its neighbors, every wireless device must keep its receiver operating at all times. This increases the power consumption of the wireless devices, which is particularly problematic for battery-powered devices.
Alternative methods have therefore been developed in which some of the wireless devices act as relay nodes while others act only as end nodes, and packets are relayed from one end node to another exclusively through relay nodes. In Japanese Patent Application Publication No. 2009-55301, for example, Noma et al. describe a wireless sensor network system in which each end device communicates with only one relay node, and the end devices conserve power by turning off their receivers except at scheduled times.
This system, however, is still imperfect. One problem is that power consumption is reduced only at the end devices, and not at the devices that act as relay nodes. Another problem is that reliability is sacrificed, because many of the possible communication links in the system are unavailable. For example, an end device cannot replace a failed communication link to a relay node by routing packets through another end device, even if the other end device is within communication range.